1. Field of the Invention
The invention relates generally to formation logging using a sonic tool. More particularly, this invention relates to methods and apparatus for sonic logging that can provide accurate determination of mud slowness.
2. Background Art
Sonic well logs are typically derived from measurements made with tools suspended in a mud-filled borehole by a cable. The tools typically include a sonic source (transmitter) and a plurality of receivers in a receiver array. The receivers in the receiver array are typically spaced apart by several inches or feet. In operation, a sonic signal is transmitted from one longitudinal end of the tool and received at the other, and measurements are made every few inches as the tool is slowly drawn up the borehole. The sonic signal from the transmitter or source enters the formation adjacent the borehole, and the arrival times and other characteristics of the receiver responses are then used to find formation parameters.
Sonic logs commonly used in the art include the slowness-time coherence (STC) log. Details of the techniques used to produce an STC log are described in U.S. Pat. No. 4,594,691 issued to Kimball et al. (the '691 patent), as well as in Kimball, et al., “Semblance Processing of Borehole Acoustic Array Data,” Geophysics, Vol. 49, No. 3, (March 1984), pp. 274–281. The '691 patent is hereby incorporated by reference in its entirety. In accordance with a method disclosed in the '691 patent, a set of time windows is applied to the compressional, shear, and Stoneley waveforms collected by an array of receivers. The time windows are determined by two parameters: the assumed arrival time at the first receiver, and an assumed slowness. For a range of arrival times and slowness, a scalar semblance is computed for the windowed waveform segments by back-propagating and stacking the waveforms and comparing the stacked energies to the unstacked energies. The semblance may be plotted as a contour plot with slowness and arrival times as the axes, with maximum semblance values indicating the determined form ation slowness value.
The STC log disclosed in the '691 patent works well for non-dispersive waves, but it is not optimal for dispersive waves. U.S. Pat. No. 5,278,805 issued to Kimball (the '805 patent) disclosed an improved method that is particularly suitable for dispersive wave analysis. This method is referred to as the dispersive slowness-time-coherence (DSTC) method, which may be used to process quadrupole signals for formation shear slowness from LWD sonic tools. See Kimball, Geophysics, Vol. 63, No. 2, March–April, 1998. The DSTC method is a model-based approach in which a set of model dispersion curves are used to determine which model dispersion curve maximizes the semblance of the back-propagated signals. DSTC analysis typically uses a concentric cylindrical layer model to represent an LWD or wireline sonic tool centered in OLE—LINK2a fluid-filled borehole within a uniform formation OLE—LINK2. However, the method does not have to use a simple concentric cylindrical layer model. If necessary, more complex model may also be used.
The formation shear slowness is one of the model parameters that are used to generate the set of dispersion curves. In accordance with the DSTC method, once the best match dispersion curve is found, the formation shear slowness is determined from the best match dispersion curve. However, the model dispersion curves depend not only on the formation shear slowness (DTs), but also on nine other model parameters: formation compressional slowness (DTc), formation density (ρb), mud slowness (DTm), mud density (ρm), hole diameter (HD), the equivalent outer diameter of the tool (OD) assuming the tool ID is fixed, collar density (ρst), collar compressional slowness (DTc—st), and collar shear slowness (DTs—st). The DSTC method as disclosed in the '805 patent assumes all these nine parameters are known and uses them to generate a set of dispersion curves as a function of formation shear slowness, DTs. The first five of these nine parameters are related to the formation and borehole properties, while and the last four parameters are related to the collar properties. For a given collar size, the collar parameters are constants, which can be measured or pre-calibrated. On the other hand, the formation/borehole parameters are variables, changing from depth to depth and from well to well. The variable formation/borehole parameters can affect the accuracy of the formation shear slowness (DTs) determined by the DSTC method.
Among the formation/borehole parameters, the mud slowness (DTm) has been found to have the most impact on the accuracy of shear slowness (DTs) calculated from the DSTC processing. Without accurate determination of the mud slowness (DTm), it is difficult to obtain accurate shear slowness (DTs) with DSTC processing, even if high quality measurement data (waveforms) with good signal-to-noise ratio are available. Therefore, accurate determination of mud slowness (DTm) is of prime importance and should be included in any sonic tool design. Otherwise, a sonic tool my not be able to provide accurate formation shear slowness measurements.
Mud slowness (DTm) has many other utilities, in addition to its use in the calculation of slow shear in quadrupole logging. For slow shear logging using dipole sources, the mud slowness is also needed in a way similar to that for quadrupole logging described above. For monopole source (for compressional (P) and fast shear (S)) logging), mud slowness (DTm) is used to set the slowness ranges for the P and S waves. These slowness ranges are used to guide the labeling algorithm in sonic log analysis. Mud slowness is also needed for computing the compressibility of borehole fluids in borehole mechanics and producibility applications.
Several approaches are possible for the measurement of mud slowness. One obvious way is to design a dedicated sensor to measure the downhole mud slowness directly. Such a sensor (sub-system) typically includes an ultrasonic pulse-echo measurement system exposed to the mud, e.g., on the outside of a drill collar. However, such sensors often cannot withstand the harsh conditions in the downhole environments. Furthermore, the high viscosity muds may include large sized rock cutting, which may result in the dispersion of sonic energies and render it difficult to ascertain whether the slowness measured at ultrasonic frequencies is the same as that measured at sonic frequencies.
Therefore, there exists a need for better methods and apparatus for the determination of mud slowness.